This invention relates to a light detector system and more particularly to a light detector system including a photo-diode and a current-mirror amplifier.
The sensing of light intensity by means of photo-diodes normally requires the use of an amplifier to enlarge the very small photo-diode signals to usable levels. Amplification is especially necessary when silicon photo-diodes are used. Photo-diode transducers are normally operated in a reverse voltage bias mode and generate a current that is a function of the intensity of light illuminating the photo-diode junction. Silicon photo-diodes are characterized as having a sensitivity, namely the ratio of output current to incident light intensity, that over a wide range of light intensities is much more linear but much smaller than the sensitivity of the more commonly employed photo-diodes made of cadmium sulphide or of III-V semiconductor compounds. Also, silicon photo-diodes are capable of being mass produced with much greater uniformity, reproducibility, and predictability of characteristics compared with the aforementioned high gain photo-diodes. The relatively low gain silicon diode almost always requires the use of an amplifier in practical light sensing applications.
It is known to use an FET type operational integrated circuit amplifier in conjunction with a photo-diode for determining appropriate shutter speed and aperture openings in cameras. It is also known to make a light summing system using a standard Miller integrating circuit by connecting the photo-diode in parallel with the Miller feed-back capacitor around the FET amplifier in a camera light monitoring system.
Conventional photo-diode amplifiers exhibit non-linear gains at the very low input signal levels generated by silicon photo-diodes operating in a low ambient light level. This is particularly true of bipolar operational amplifiers that are characterized as having an input offset current that makes them unsuitable for providing reliable and constant gain of such small signals.
In addition, FET integrated amplifiers generally require substantially more semiconductor surface area, or real-estate, than for a comparable integrated bipolar amplifier; so for economy the latter is preferred. When it is considered that to include a silicon photo-diode on the same semiconductor chip with an associated integrated circuit amplifier would provide further economy, it is found that combinations of separate elements of the old art do not offer of suggest practical low cost solutions.
It is accordingly an object of the present invention to overcome the aforementioned shortcomings of the prior art.
It is another object of the present invention to provide a simple bipolar semiconductor integrator for use with a silicon photo-diode.
It is another object of the present invention to provide a low cost integrated circuit light detector system including a silicon photo-diode and an amplifier for effectively sensing light intensity over several orders of magnitude.
It is yet a further object of this invention to provide an integrated circuit including a photo-diode and an amplifier for use in automatically controlling light exposure in a camera.